1. Field of the Invention
The invention relates to positioning systems for railways. Such devices measure the absolute and relative position and speed of railway vehicles and supply their measured values to driver displays, signalling, traction control systems and other users. In the railway context, absolute positioning refers to preset mile or kilometer positions on a track which is recorded in files and on wayside milestones. Relative positioning refers to a distance travelled since an earlier point in time.
2. Description of Related Art
Known solutions for relative positioning apply wheel rotation measurements, see e.g. GB388761, Radar, see e. g. U.S. Pat. No. 4,791,424 and induced magnetic fields measurements. Known solutions for absolute positioning apply wayside tags in the form of electronic transponders, see e. g. EP1813499 or track cable crossing locations, see e.g. EP0593910. The need to provide 2 separate systems for absolute and relative positioning drives cost and the amount of hardware to be installed. Satellite positioning combines absolute and relative positioning, see e. g. DE19731110 A1. However, data availability in tunnels and narrow valleys is low precluding its use as a universal solution. Magnetic speed measurement devices using pulsed coils to create magnetic markers in the rail are known from DE2164312 and FR-A-2673901. As shown in WO 01/66401 A1, absolute and relative positioning have been combined in one system using a speed measurement device measuring induced magnetic fields known from U.S. Pat. No. 5,825,177 which recognises patterns in the track like the rail gaps at points. This method only has limited coding opportunities and due to the similarity between points, dependability is not optimised.